The biodegradable PLA resins have good mechanical properties physical properties mechanical properties as well as biodegradable property and can be degraded in natural environment and be transformed into water and carbon dioxide without harming other organisms. Therefore, PLA resins have attracted a lot of attention in many fields such as medical materials and the substitution of conventional resins.
Polyesters of PLA and their copolymers of the present invention include PLA, PGA, PLA/PGA, PLA/PEG, poly (lactic acid-co-amino acid), PLA/CL, poly (lactic acid-co-Chitosan) or poly (lactic acid-co-ethyl propyl acrylamide), all of which have carboxyl groups and active hydrogen groups in the same molecular. Since these compounds are prepared by deglycolation of the ending hydroxide groups of polyester with low molecular weight, the concentration of the ending groups decrease significantly with the increase of molecular weight, and the increase of molecular weight is limited by the decomposition reaction. Therefore, it is difficult to synthesize a polymer with high enough molecular weight to form a tough film by common deglycolation reaction. In other words, the molecular weight of poly (lactic acid)-based resin prepared by current methods is too low to form practical films. Additionally, the production equipment and production cost are also a problem because of the rigorous reaction condition of high temperature and high vacuum.
Japanese patent 7-228675 discloses a method of producing a polyester with high molecular weight. This method includes a series of processes: first, polyol and polyacid are heated in an organic solvent; second, the generated water is distilled from the organic solvent; third, the organic solvent is dehydrated by a desiccant and then circulated into the reaction system. Through the above processes, the dehydration condensation polymerization takes place. However, this reaction requires high temperature and high vacuum which result in high cost of producing equipment.
JP59-96123 discloses a PLA preparation method by direct condensation polymerization during decompression at high temperature in the presence of catalyst. However, the molecular weight of the PLA is very low and the PLA can not be used widely. U.S. Pat. No. 4,273,920 indicates that the molecular weight of PLA can be up to 30,000 through direct condensation polymerization with high acid ion exchange resin. Although the cost of direct condensation polymerization is low, the molecular weight of the obtained PLA is still too low to meet the demand of industrial application.
A currently known preparation method is a polymerization of dicarboxylic acyl chloride and diol, or a dehydrochlorination reaction of hydroxyl acyl chloride. The high selectivity in the chlorination process of carboxyl has been reported and when thionyl chloride is used as a chlorinating agent, the needed hydroxylic amic acyl chloride can be obtained by esterification between dicarboxylic acid and diatomic alcohol or esterification of hydroxamic acid containing phenyl.
It is also known that hydroxylic amino acyl chloride, and the acyl chloride of lactic acid or its oligomers are unstable, indicating that the existence of such chloride are not completely known. JP4-3763 describes a method of synthesis of PLA, PGA and their copolymers with high molecular weight, wherein the oligomer of PLA or PGA was first reacted with thionyl chloride, oxalyl chloride, succinyl chloride or paraphthaloyl chloride followed by polycondensation. CN 1071340C describes a method of synthesis of PLA utilizing halogenating imidazole onium salt. Lactic acid or its oligomer was first reacted with halogenating imidazole onium salt to obtain acyl chloride and the PLA was synthesized through a dehydrochlorination reaction.
The synthesis of acyl chloride of PLA or its low polymer, however, has not been confirmed, and the obtained polymer has low degree of polymerization. Thus there may be little chance in transforming the carboxyl into acyl chloride by these chlorinating agents.
CN1102583C provides a method of synthesizing a thiaole cation condensing agent which has many virtues such as easy preparation, ready availability of raw material, good stability at room temperature, high re-activity and desirable-optical purity.